1. Field of the Invention
An embodiment of the invention disclosed in this specification and the like relates to a semiconductor device and a method for manufacturing the semiconductor device.
Note that a semiconductor device in this specification and the like refers to any device that can function by utilizing semiconductor characteristics, and for example, electro-optical devices, image display devices, semiconductor circuits, and electronic devices are all semiconductor devices.
2. Description of the Related Art
A technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. Such a transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (also simply referred to as a display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a technique by which a transistor is formed using zinc oxide or an In—Ga—Zn-based oxide semiconductor as an oxide semiconductor is disclosed (see Patent Documents 1 and 2).
Non-Patent Document 1 discloses a transistor including a stack of oxide semiconductors. However, in the structure disclosed in Non-Patent Document 1, the oxide semiconductor functioning as a channel is in contact with a silicon oxide film; thus, silicon, a constituent element of the silicon oxide film, might be mixed into the channel as an impurity. The impurity mixed into the channel causes degradation of the electrical characteristics of the transistor.